The present invention is directed to a system and method for determining engine control parameters such as airflow and/or fuel flow based on engine torque.
Electronic airflow control systems, such as variable cam timing systems and electronic throttle control systems, replace traditional mechanical throttle cable systems with an xe2x80x9celectronic linkagexe2x80x9d provided by sensors and actuators in communication with an electronic controller. This increases the control authority of the electronic controller and allows the airflow and/or fuel flow to be controlled independently of the accelerator pedal position.
To control the actual output engine brake torque to achieve the driver demanded engine brake torque, it is necessary to determine appropriate values for corresponding engine control parameters, such as airflow and fuel flow. Preferably, this computation accounts for variations in engine operating parameters, such as engine operating temperature and accessory losses.
Prior art approaches convert the desired engine torque to a desired airflow using a two-dimensional lookup table with inputs for desired torque and engine speed. However, such lookup tables are typically calibrated for stoichiometric operation and for maximum brake torque (MBT). The resulting desired airflow is then modified to account for engine operating temperature and air/fuel ratio to generate a modified or corrected airflow. The corrected desired airflow is then achieved via an appropriate airflow rate controller. This approach may be seen by reference to U.S. Pat. Nos. 5,351,776, 5,383,432 and 5,501,989, for example.
While the prior art approaches may be acceptable for many applications and operating conditions, it is desirable to provide a more robust engine torque controller which improves powertrain performance for current applications and is more amenable to new engine technologies and control strategies such as direct injection, lean burn, variable cam timing, and variable displacement applications.
It is an object of the present invention to provide a system and method for determining at least one engine control parameter based on a requested engine brake torque which has been compensated or modified to account for current engine operating parameters and/or control modes.
Another object of the present invention is to provide a system and method for engine torque control capable of simultaneously determining a desired airflow and fuel flow to produce a desired engine torque that accounts for engine operating parameters.
In carrying out the above object and other objects, advantages, and features of the present invention, a system and method for determining control parameters for an engine include modifying a first engine torque based on estimated losses associated with a previously determined desired engine load to determine a second engine torque, modifying the second engine torque based on a desired ignition angle and air/fuel ratio to determine a third engine torque, determining a desired engine load based on the third engine torque, and converting the desired engine load to at least one engine control parameter.
The present invention provides a number of advantages over prior art control strategies. The present invention recognizes that the transformation between torque and airflow is an affine transformation rather than a linear transformation. While this distinction may result in imperceptible changes in operation for combustion at or near the stoichiometric ratio, it becomes problematic for various applications including lean burn strategies where air/fuel ratios are far from stoichiometry (around 18:1).
The present invention also recognizes the significant interrelations between various engine control parameters such as air/fuel ratio and ignition timing by determining control parameters after the desired torque has been fully compensated for losses. In addition, the present invention accounts for the reduction in secondary pumping effects which may reduce pumping losses as much as 50% during lean operation or variable cam timing operation. In one embodiment, the present invention provides improved torque control during transient conditions, such as during a gear shift or during changes in operator demand, by determining fuel flow and airflow simultaneously based on the desired torque rather than determining fuel flow as a result of actual airflow.
The above advantages and other advantages, objects, and features of the present invention, will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.